Support .mrq file when .frq isn't present [MOD+]

[Astel123457]

While it is possible in theory to write to .mrq, just reading it is enough for now, since mrq is a semi complicated format that acts more like a database rather than just a simple array of f0 data points like frq.

[Copilot]

Pull Request Overview

This PR adds support for loading MRQ (Moresampler) frequency files as an alternative to FRQ files in OpenUtau. MRQ files are centralized frequency data files stored in a desc.mrq file in the voicebank directory, containing pitch information for multiple audio files.

• Implements a new Mrq class that implements the IFrqFiles interface to read moresampler frequency data
• Adds MRQ file loading to the frequency file fallback chain (tries FRQ first, then MRQ)
• Adds utility method GetMrqFile to locate the centralized desc.mrq file

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
OpenUtau.Core/Classic/VoicebankFiles.cs	Adds GetMrqFile method to resolve the path to the centralized desc.mrq file
OpenUtau.Core/Classic/Frq.cs	Implements the Mrq class for parsing binary MRQ files and integrates it into the frequency file loading chain

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